Because he is... Hashirama; "please stop, Madara" Hint hint... Even Orochimaru said sauske will surpass Madara, maybe we'll see just how in the upcoming manga chap.
Unless Madara isn't the final villain. It may seem strange but I have a feeling that Zetsus could possibly be the final villain. Looking at the conversation between Madara and Zetsus in the last chapter it seems clear that they are just allies without much respect for each other other than master and puppet. Zetsus could possibly be using Madara to bring the ten tails back to full strength and then take over when the time is right. I know it sounds odd since Zetsus seems like a weakling but the fact that Zetsus was created from the Gedo Mazo and the Gedo Mazos main purpose was to get back all the chakra that was stolen from it then Zetsus 's plan could be the same seeing that it's basically just a part of the Gedo Mazo.